Application Note

Use of Synthetic Graphite in High Performance RFand Microwave PCBsJohn Priday,
Teledyne Labtech

A Teledyne Defense Electronics Company

TDE Application Note

Use of Synthetic Graphite in High Performance
RF and Microwave PCBsJohn Priday, CTO
Teledyne Labtech

Introduction
The continued advances in miniaturization of electronic devices increases the challenges in managing waste heat. With active
semiconductor devices becoming significant
sources of heat, the printed circuit board (PCB)
Self-adhesive
synthetic
is asked to play multiple roles – to supply
graphite sheet
power to the device; to pass the increasingly
high-speed signals between devices withRF Section
as little loss as possible; and also, to play asignificant part in passing heat energy awayfrom such devices. In recent consumer
electronics such as smartphones, the taskof heat dissipation is not left solely to thePCB; areas of high heat concentration are
also managed using pre-formed adhesive
pads made from synthetic graphite, a gooddissipator of heat. These are used to spreadthe heat energy over a wide area, reducing
the temperature-induced stress on individual
devices such as power amplifiers, improving
life expectancy. Synthetic graphite has someFigure 1: Typical smartphone RF section showing use of synthetic graphite pad to aid thermal
appealing properties as we’ll see later.
management
Some of the most acute heat management challenges are seen in aerospace and defence applications. Usually these have anextreme focus on system size, weight, and power (SWaP), which makes heat management an even more critical topic. Suchapplications will typically sacrifice low cost to achieve the highest performance combination of thermal management, signal
fidelity and size/weight. Examples include power amplifiers, phased array transmit/receive (Tx/Rx) modules and high speed
digital processing cards. The most effective heat dissipation technique we’ve found for power devices is the use of copper coins
which we’ve described elsewhere1. However, we wondered whether the benefits of synthetic graphite could be leveraged for
SWaP-constrained medium power applications. Our concept was that PCBs use sheets of copper as ground planes. Could thesebe replaced with synthetic graphite instead?

1Application Note - Thermal Management in High Performance RF and Microwave PCBs
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TDE Application Note

Use of Synthetic Graphite in High Performance RF and Microwave PCBs

Why Care About Synthetic Graphite?
Synthetic graphite has an excellent in-plane thermal conductivity of between 1500 to 1600 W/mK (approximately 4 times that ofcopper) and densities between 2.0 to 2.1 g/cm3 (approximately one quarter of copper). In other words, it’s 4 times lighter thancopper, and transfers heat 4 times better. It is available in thin sheets ranging from 10 μm to 40 μm in thickness and is typicallysupplied with a self-adhesive coating and on a carrier. What’s not to like? The answer is two-fold: It is great at spreading heat
across its width (x, y) but very poor at transmitting heat in its third dimension (z), for example down through the thickness ofa PCB; a typical z-plane conductivity is 5 W/mK. More significantly it is very slippery – graphite is often used as a dry lubricant.
This makes bonding it into a reliable PCB structure very difficult, akin to trying to glue a wet bar of soap. Given that many highperformance PCBs can use four to six copper planes or more, a reduction in mass of these would help the overall weight of asystem.
Requirements for Using Graphite Planes in PCBs
High density active devices, often mounted in QFN style packages, can dissipate significant heat. One of many roles that thePCB must perform is to channel heat from the underside of the semiconductor devices through to the chosen heatsinking
scheme as efficiently and effectively as possible.

QFN Package
Clamped Cold WallHeat Spreading
Laterally ThroughBoardSynthetic Graphite
Sandwiched Inside PCB

Figure 2: Aerospace card application with a QFN packaged high power device on a PCB. Thermal management is aided through use of cold walls andboard clamps to allow heat to be moved into the enclosure. Use of thermal graphite would be beneficial if a suitable approach could be found toaccomplish it.

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TDE Application Note

Use of Synthetic Graphite in High Performance RF and Microwave PCBs
This is typically achieved through mounting devices on thermal/ground pads on the top layer of the PCB. These are thenelectrically and thermally connected to lower layers through plated through-hole (PTH) vias. Sometimes these are filled withsolder, but more often with thermally conductive filler.
To replace copper planes in the PCB sandwich, graphite would have to perform well in several areas to be worth pursuing:
1.
Have enough thermal advantage to be worth the effort.
2.
Not add weight.
3.
For the graphite planes to not significantly impact the passage of microwave signals in their role as ground planes.
4.
To be mechanically robust and survive normal processing stresses during manufacture and use.
5.
To be easy to process using normal PCB manufacturing techniques.
Development of Graphite-Layer PCBs
Of the needs listed above, the mechanical robustness requirement was by far the most difficult. We tried many different
techniques to get the graphite layer to stick. Often thermal conductivity performance was good, but thermal stress tests would
induce cracked vias and/or delamination. Patient development efforts eventually resulted in a successful approach, and it is
the results of this that are described below. These use an evaluation PCB designed to allow each aspect of performance to be
demonstrated.
Thermal Test Structure

Microwave Test Structures
Stripline(A)Microstrip(B)

(C)

7mm

100mm

11mm

Figure 3: Test board designed to evaluate thermal conductivity performance (A), and microwave
signal performance (B), (C).

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TDE Application Note

Use of Synthetic Graphite in High Performance RF and Microwave PCBs
Test PCB
An overview of the evaluation PCB is shown in Figure 3. The board was produced in two versions, one using two graphite
planes, the other nearly identical but using copper planes. These are illustrated in Figure 4, with more detail given in theappendix.

Thermal Test Pad(A)Stripline(B)Microstrip
(C)

40 μm
Graphite
PlanesFilled Vias

35 μm
CopperPlanes

Figure 4: Illustration of the two nearly identical test boards, one using synthetic graphite planes, theother substituting copper planes.

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TDE Application Note

Use of Synthetic Graphite in High Performance RF and Microwave PCBs
Experiment 1: Is it Rugged?
The most extreme stress experienced by almost all PCBs is during processing. The thermal shocks of going through solder
processing steps are extreme. Customers usually require PCBs to withstand two excursions from room temperature up to solder
temperature for double-sided boards, then to allow for the possibility of three more re-work cycles in manufacture or repair.
This is captured in IPC-650-TM-2.6.8 Condition A. Tests were performed with 1 cycle of 10 seconds solder float @ 288 °C andthen 5 cycles, starting from ambient room temperature for each cycle. All tests passed. Figure 5 shows cross-sections taken
post thermal stress. The only notable degradation seen was in the via filler that cracked after 5x thermal stresses (see figure 8),most likely a result of CTE (coefficient of thermal conductivity) mismatch. This was not deemed significant.
Pad

Graphite Plane

Graphite Plane

Filled Via

Pad

1x cycle

Pad

FillerCrack
5x cycle

Figure 5: Cross-section of the graphite-plane PCB after thermal stress, after 1 cycle (above) and after
5 cycles (below).

Having survived the worst extremes of processing, boards will also withstand the rigors of real use.

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TDE Application Note

Use of Synthetic Graphite in High Performance RF and Microwave PCBs
Experiment 2: Can it Dissipate Heat?
Obviously, this is a wasted effort if the heat dissipation effectiveness is not significant. To explore this, a power resistor wasmounted onto both PCBs using H20E conductive epoxy to act as a heat source. Thermal measurements were taken using aTeledyne FLIR infrared camera, as noted in Figure 6. In both cases the same power was applied (6.7 W) and allowed to stabilize
for 5 minutes prior to taking any readings. The boards were suspended in free air with no additional heatsinking.

CopperPlanes

GraphitePlanes

Figure 6: Thermal dissipation experiments. The test PCB with graphite planes (lower) showedsignificantly better dissipation.

As can be seen in Figure 6, the resistor was more than 20 °C cooler in the graphite-plane case compared to the equivalent using
copper planes. This would significantly improve the Mean Time Between Failure (MTBF) reliability of a semiconductor device innormal use.

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TDE Application Note

Use of Synthetic Graphite in High Performance RF and Microwave PCBs
Experiment 3: Does it Still Pass Microwave Signals?
Many high-performance PCBs also move wide bandwidth signals around, whether they are designed for use in signal chain
applications, or, increasingly, high speed digital processing. It is therefore important for this new heat mitigation material to also
not degrade signals significantly.
To evaluate microwave performance, our test structures had stripline (B) and microstrip (C) test paths. To make measurements,
10 mm aluminium plates were bonded onto the back of the two PCBs to allow mounting of SMA connector interfaces.
Microwave Test Structures
Metal Block for Connector Mounting(A)

Stripline

Microstrip

(B)

(C)

Figure 7: Graphite-plane PCB shown with test connectors and aluminum mounting block.

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TDE Application Note

Use of Synthetic Graphite in High Performance RF and Microwave PCBs
A Vector Network analyser was used to conduct measurements over a frequency range of 5 to 20 GHz. The results are shown in
Figure 8 for both stripline (B) and microstrip (C) test structures. The graphite planes caused a maximum degradation in insertion
loss performance of, at most, 2 dB. Note that this was for a 100 mm trace length, which is two- to three-times a typical total
length of signal path on most PCBs we see. For many applications, we believe this performance will be more than acceptable.
(Results above 18 GHz for the stripline case were not optimal due to a mismatch at the connector interface.)

Insertion Loss (S21) for 100mm Trace Length0CopperdB-5Graphite-10-15-20

Stripline (B)5Microstrip (C)10152051015GHz20Figure 8: Measurements from 5 to 20 GHz of insertion loss (S21)A Teledyne Defense Electronics CompanyPage 9 of 11

TDE Application Note

Use of Synthetic Graphite in High Performance RF and Microwave PCBs
Summary
We set out to investigate whether the replacement of copper planes in PCBs with synthetic graphite was both desirable, andpractical. Using our list of criteria from earlier, we conclude the following:
1. Graphite-plane PCBs do have enough thermal advantage to be worth the effort, keeping a component more than 20 °C
cooler in our example experiment.
2. Graphite-plane PCBs do not add weight, and actually weigh less, particularly when more ground plane layers are employed.
3. Such boards do not significantly impact the passage of microwave signals in their role as ground planes, showing less than
2 dB impact compared to copper for 100mm path lengths as both stripline and microstrip. As this is 2x to 3x normal total
signal path lengths we encounter, we believe this will be acceptable in many applications.
4. These boards are mechanically robust and survive normal processing stresses during manufacture and use, proven throughIPC-650-TM-2.6.8 thermal shock tests, even after 5 cycles from room temperature to 288 °C in 10 seconds each time withno cracked vias or layer delamination. They operate over the same temperature ranges as regular PCBs.
5. The new boards use normal PCB manufacturing techniques, with only a modest increase in time and cost.
Synthetic graphite offers a significant weight saving and offers benefits in terms of thermal management for PCBs that havemedium power devices mounted on them. It’s ability to carry and distribute heat efficiently in the X-Y plane, that can be takenout to the perimeter of the PCB and then extracted via clamps to a cold wall will be attractive for some applications. The
additional benefit of allowing semiconductor devices to operate at substantially reduced stabilised temperatures will also enableimproved MTBF predictions for PCBAs.
There are certain restrictions as a result of the necessity to maintain via integrity and we would strongly recommend early stage
discussions with Teledyne Labtech to ensure all design for manufacture considerations are included.
Acknowledgements
KANEKA Corporation1 for kindly supplying the synthetic
graphite sheets GraphinityTM used.

1John Priday, Chief Technical Officerof Teledyne Labtech, developed
Labtech’s microelectronic assembly(chip and wire) facility and oversees all
microwave testing services. He started
his career with Marconi Radar before
moving the Marconi Research Centre
in Great Baddow.

KANEKA | Electrical and electronic materials
A Teledyne Defense Electronics CompanyPage 10 of 11

6th January, 2022TDE Application Note

Use of Synthetic Graphite in High Performance RF and Microwave PCBs
Appendix – Test Board Construction
For our evaluation we selected 40 μm sheets of synthetic graphite that have a thermal conductivity of 1500 W/mK and a densityof 2.0 g/cm3. Figure 9 shows the construction layers we used. For the copper equivalent board, the graphite planes werereplaced with 35 μm copper planes.

Figure 9: Construction of the synthetic graphite test board showing layer materials
and thicknesses (mm)A Teledyne Defense Electronics CompanyPage 11 of 11